1. Field of the Invention
An automatic focus adjustment device that automatically adjusts focussing corresponding to the velocity of a moving subject.
2. Description of Related Art
A conventional automatic focus adjustment device performs automatic focus adjustment relative to moving subjects.
FIG. 10A is a diagram showing focus adjustment by one conventional type of automatic focus adjustment device. The horizontal axis represents time and the vertical axis represents the scrolling amount of the shooting lens given a predetermined position, for example, infinity as a reference point. The subject image trace is the ideal lens scrolling position trace if the device maintains continuous focus on a moving subject. The lens focus position trace is the trace of an actual scrolling amount of the lens position.
With this type of automatic focus adjustment device, the lens scrolling mechanism monitors, as an error control, the focus shift amount (hereafter defocus amount) of the shooting lens and the change in the defocus amount with time. The defocus amount is the amount the lens focus position trace follows the subject image trace,
When a release button is depressed at time Tnow, the controlled positioning of the lens scrolling mechanism focus position is changed by the amount DEFx. The defocus amount DEFx at the exposure time Tex is determined using a linear approximation based on the defocus amount computed prior to release and the amount of change in the defocus amount with time. In this way, the focus is adjusted to the predicted position of the subject image at the exposure time, Tex.
With this type of conventional automatic focus adjustment device, a certain amount of lens scrolling mechanism deceleration time is needed. The deceleration time halts the lens scrolling mechanism with a high degree of precision at the predicted position of the subject image. Consequently, the desired driving amount cannot be achieved within the limited length of time (from Tnow to Tex) for a subject with a fast movement velocity. Thus, it is impossible to effect focussing at the exposure time Tex for a high velocity subject, as shown in FIG. 10B.